This invention relates to an improved graphic forms overlay apparatus for use with non-impact printers whose operation requires imaging on a movable light sensitive medium.
Laser-xerographic printers are one type of non-impact printer well known in the art (see U.S. Pat. Nos. 3,659,936 and 3,698,006). Laser-xerographic printers utilize a modulated beam of laser light which is caused to sweep periodically across a photoconducting drum. The information represented by the modulated light is thereby temporarily stored as electrostatic charges on the surface of the drum. In some laser-xerographic printers, the electrostatic image is developed with toner and the toned image is then transferred to paper to form a permanent record.
It is often desirable to print data such as alphanumeric characters on a government, business or graphic form. Rather than purchase expensive preprinted forms, it is possible to print these forms at the same time that the data is printed. An example of one approach for accomplishing the above using a laser printer is disclosed in U.S. Pat. No. 3,864,697 at column 4, lines 1-14. Another approach used with laser-xerographic printers having cylindrical photoconducting drums comprises a negative of a graphic forms pattern mounted on a hollow, cylindrical rotatable forms drum. The forms drum is equipped with a linear light source fixedly mounted within the hollow forms drum. The photoconducting drum and the forms drum are mechanically coupled so that the forms drum rotates therewith in order to provide synchronous integrated printing of the graphic forms data and information data. Mechanical interconnections can be accomplished by mounting the forms drum above the printer, for example, with the axis of the forms drum parallel to the axis of the photoconducting drum. A belt and pulley is used to connect the ends of the two drums so that when one drum turns the other also turns. As the forms drum rotates, light transmitted through the graphic forms negative causes a light image of a linear portion of the graphic form pattern to be directed onto that linear portion of the photoconducting drum being swept by the laser at that time. Typically, an optical system of lenses and mirrors is used to direct the light images from the forms drum to the photoconducting drum.
When it is desired to retrofit a graphic forms overlay apparatus, like the mechanically coupled forms drum described above, to an existing laser-xerographic printer, it is necessary to mount the forms drum in proper position as described above. For example, the forms drum is mounted above the printer, and the drive belt coupling the two drums is passed through an opening in the top of the printer and around an end of the photoconducting drum.
To insure proper vertical alignment of the graphic forms image with the information data when printed, an indicator means for circumferential alignment of the graphic forms negative on the forms drum must be provided on the forms drum. On some overlay devices, this includes a pair of protrusions on the forms drum lying along a line parallel to the axis of the drum. Graphic forms negatives are made with two spaced apart alignment holes which receive the two protrusions on the forms drum on the graphic forms negative when it is placed on the drum. This method of alignment requires that the holes be placed accurately on the graphic forms negative and that the initial rotational position of the forms drum have a predetermined relationship with the initial position of the photoconducting drum. If the holes in the graphic forms master are not properly positioned from one graphic forms master to the next, or, if the rotational relationship of the two drums is disturbed proper alignment of graphic forms images and information data being printed will be absent.
An alternate known means of providing synchronized rotation between the forms drum and photoconducting drum requires the use of synchronous motors for both drums. The two synchronous motors rotate continuously in phase with one another at the same rate. The synchronous motors may be connected to the drums through gearing arrangments in order to compensate for possible differences in diameter between the two drums. An alignment mark located on the graphic form is detected by a detector once every revolution of the forms drum, and the detector transmits a top-of-forms signal to the printer which starts a page of printing. Hence, the graphic forms image is aligned with the data image from the printer on the photoconducting drum. However, the printer must wait for the signal from the forms drum before printing can be initiated. In some laser-xerographic printers the photoconducting drum is fabricated with a seam running along the surface of the drum parallel to the axis of the drum. No imaging of data can take place along the seam or a small region surrounding the seam. With the alignment procedure described above for synchronous motor drives of the photoconducting and forms drum, care must be taken when selecting and mounting a graphic forms negative on the forms drum so that printing is not started which will result in overlaying of the seam.